On January 6, 1993, a half-hour after exposing his genitals at a tanning salon in Johnson County, Kansas, Michael Crane repeated the show at a video store. This time, after waiting until he was alone with the clerk, he grabbed her by the neck, ordered her to perform oral sex, and threatened to rape her before abruptly leaving.

Crane was convicted of lewd and lascivious behavior for the incident at the tanning salon and aggravated criminal sodomy, attempted rape, and kidnapping for the assault at the video store. He received a sentence of 35 years to life.

In 1996 the Kansas Supreme Court overturned the assault-related convictions. Rather than go through another trial, the prosecutor let Crane plead guilty to aggravated sexual battery, which carries a lighter sentence. He consoled the victim by telling her the state would seek to confine Crane indefinitely under the Kansas Sexually Violent Predator Act.

That is what happened in 1998, when Crane was about to be released on parole after serving four and a half years in prison. At Crane's commitment trial, the jury concluded that he suffered from "antisocial personality disorder combined with exhibitionism," making it likely that he would commit further acts of sexual violence.

Under the Kansas statute, which is similar to laws in about 15 other states, this determination was enough to lock Crane up until he's "cured"--or for the rest of his life, if his condition should prove intractable. Although it upheld the Kansas law in 1997, the U.S. Supreme Court recently decided that Crane's commitment was unconstitutional.

Why? Not because the state is seeking to punish Crane twice for the same crime by calling his imprisonment "treatment." Not because the "disorders" from which he is said to "suffer" are figments of the psychiatric imagination. No, the only problem, so far as the Supreme Court is concerned, is that the jury at Crane's commitment trial did not explicitly find that he has trouble controlling his behavior.

"Inability to control behavior will not be demonstrable with mathematical precision," the Court conceded. In fact, it will not be demonstrable at all. While it is possible to prove that a defendant did not resist his antisocial impulses, it can never be shown that he could not. In any case, the idea that Crane can't help himself is inconsistent with the assumption of responsibility that made it appropriate to punish him in the first place.

An offender's dangerousness, another element of the justification for commitment, is likewise difficult to assess. Psychiatrists are notoriously bad at predicting who will commit further crimes, and there appears to be no basis for the commonly held belief that sex offenders are especially likely to do so.

In truth, the "diagnosis" that made Crane's continued imprisonment possible was simply a matter of applying pseudoscientific labels to the same actions for which he had already been punished. His "lewd and lascivious behavior" became "exhibitionism." His violence became "antisocial personality disorder," defined as "a pervasive pattern of disregard for and violation of the rights of others."

In its decision, the Court cited an estimate that "40%-60% of the male prison population" would qualify for the latter diagnosis. It also noted that the "concept of mental disorder lacks a consistent operational definition" and that psychiatry is "an ever-advancing science"--a kind way of saying that psychiatrists are making it up as they go along.

Although prosecutors usually resist psychiatry's medicalization of behavior because it tends to erode individual responsibility, they're happy to take advantage of it when they can. At Crane's commitment trial, the prosecutor asked the video store clerk whether she was "understandably very upset" that Crane "only served a little over four years" despite his long initial sentence. "Yes, very," she said.

Despite the evidence that prosecutors resort to sexual predator laws when they're unhappy with the results of a criminal trial, the Supreme Court continues to insist that confinement in a mental institution is not a punishment. Hence the Fifth Amendment's prohibition of double jeopardy does not apply.

Likewise, the Court has abandoned its usual suspicion of imprisoning people for crimes they might commit in the future. Although keeping Crane off the streets after he has completed his legally imposed sentence certainly looks like preventive detention, the Court is sophisticated enough in matters of mental health to know that appearances can be deceiving.